The Transiting Exoplanet Survey Satellite (TESS) is an MIT-led, NASA-funded Explorer-class planet finder launched in April 2018. TESS will carry out a 2-year all-sky survey with the primary goal of detecting small transiting exoplanets around bright and nearby stars. The TESS instrument consists of four wide-field cameras in a stacked configuration, providing a combined field of view of 24 deg × 96 deg that spans approximately from the ecliptic plane to the ecliptic pole. In order to achieve the desired photometric precision necessary for the mission, TESS uses the instrument cameras as star trackers during fine-pointing mode to enhance attitude accuracy and stabilization for science operations. We present our approach in quantifying the expected performance of the fine-pointing system and assessing the impact of pointing performance on the overall photometric precision of the mission. First, we describe the operational details of the fine-pointing system with the science instrument being used for star-tracking. Next, we present the testing framework used to quantify the attitude determination performance of the system and the expected attitude knowledge accuracy results, both in coarse-fine pointing hand-off and in nominal fine-pointing conditions. By combining simulations of the instrument and the spacecraft bus, we quantify the closed-loop fine-pointing stability performance of the system in nominal science operations as well as in the case of camera unavailability due to Earth/Moon interference. Finally, we assess the impact of platform pointing stability on the photometric precision of the system using detailed system modeling and discuss the applicability of mitigation techniques to reduce the effect of jitter on TESS science data.
G. Ricker, R. Vanderspek, J. Winn, S. Seager, Z. Berta-Thompson, A. Levine, J. Villasenor, D. Latham, D. Charbonneau, M. Holman, J. Johnson, D. Sasselov, A. Szentgyorgyi, G. Torres, G. Bakos, T. Brown, J. Christensen-Dalsgaard, H. Kjeldsen, M. Clampin, S. Rinehart, D. Deming, J. Doty, E. Dunham, S. Ida, N. Kawai, B. Sato, J. Jenkins, J. Lissauer, G. Jernigan, L. Kaltenegger, G. Laughlin, D. Lin, P. McCullough, N. Narita, J. Pepper, K. Stassun, S. Udry
KEYWORDS: Exoplanets, Stars, Satellites, Planets, Cameras, Space telescopes, James Webb Space Telescope, Space operations, Charge-coupled devices, Observatories
The Transiting Exoplanet Survey Satellite (TESS) will discover thousands of exoplanets in orbit around the brightest stars in the sky. This first-ever spaceborne all-sky transit survey will identify planets ranging from Earth-sized to gas giants. TESS stars will be far brighter than those surveyed by previous missions; thus, TESS planets will be easier to characterize in follow-up observations. For the first time it will be possible to study the masses, sizes, densities, orbits, and atmospheres of a large cohort of small planets, including a sample of rocky worlds in the habitable zones of their host stars.
Very precise on-ground characterization and calibration of TESS CCD detectors will significantly assist in the analysis of the science data from the mission. An accurate optical test bench with very high photometric stability has been developed to perform precise measurements of the absolute quantum efficiency. The setup consists of a vacuum dewar with a single MIT Lincoln Lab CCID-80 device mounted on a cold plate with the calibrated reference photodiode mounted next to the CCD. A very stable laser-driven light source is integrated with a closed-loop intensity stabilization unit to control variations of the light source down to a few parts-per-million when averaged over 60 s. Light from the stabilization unit enters a 20 inch integrating sphere. The output light from the sphere produces near-uniform illumination on the cold CCD and on the calibrated reference photodiode inside the dewar. The ratio of the CCD and photodiode signals provides the absolute quantum efficiency measurement. The design, key features, error analysis, and results from the test campaign are presented.
The Transiting Exoplanet Survey Satellite, a NASA Explorer-class mission in development, will discover planets around
nearby stars, most notably Earth-like planets with potential for follow up characterization. The all-sky survey requires a
suite of four wide field-of-view cameras with sensitivity across a broad spectrum. Deep depletion CCDs with a silicon
layer of 100 μm thickness serve as the camera detectors, providing enhanced performance in the red wavelengths for
sensitivity to cooler stars. The performance of the camera is critical for the mission objectives, with both the optical
system and the CCD detectors contributing to the realized image quality. Expectations for image quality are studied
using a combination of optical ray tracing in Zemax and simulations in Matlab to account for the interaction of the
incoming photons with the 100 μm silicon layer. The simulations include a probabilistic model to determine the depth of
travel in the silicon before the photons are converted to photo-electrons, and a Monte Carlo approach to charge diffusion.
The charge diffusion model varies with the remaining depth for the photo-electron to traverse and the strength of the
intermediate electric field. The simulations are compared with laboratory measurements acquired by an engineering unit
camera with the TESS optical design and deep depletion CCDs. In this paper we describe the performance simulations
and the corresponding measurements taken with the engineering unit camera, and discuss where the models agree well in
predicted trends and where there are differences compared to observations.
The Transiting Exoplanet Survey Satellite (TESS) is an Explorer-class mission dedicated to finding planets
around bright, nearby stars so that more detailed follow-up studies can be done. TESS is due to launch in
2017 and careful characterization of the detectors will need to be completed on ground before then to
ensure that the cameras will be within their photometric requirement of 60ppm/hr. TESS will fly MITLincoln
Laboratories CCID-80s as the main scientific detector for its four cameras. They are 100μm deep
depletion devices which have low dark current noise levels and can operate at low light levels at room
temperature. They also each have a frame store region, which reduces smearing during readout and allows
for near continuous integration. This paper describes the hardware and methodology that were developed
for testing and characterizing individual CCID-80s. A dark system with no stimuli was used to measure the
dark current. Fe55 and Cd109 X-ray sources were used to establish gain at low signal levels and its
temperature dependence. An LED system that generates a programmable series of pulses was used in
conjunction with an integrating sphere to measure pixel response non-uniformity (PRNU) and gain at
higher signal levels. The same LED system was used with a pinhole system to evaluate the linearity and
charge conservation capability of the CCID-80s.
KEYWORDS: Stars, Planets, Exoplanets, Space operations, Satellites, Cameras, Charge-coupled devices, Space telescopes, James Webb Space Telescope, Observatories
The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its 2-year mission, TESS will employ four wide-field optical charge-coupled device cameras to monitor at least 200,000 main-sequence dwarf stars with IC≈4−13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from 1 month to 1 year, depending mainly on the star’s ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10 to 100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every 4 months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.
KEYWORDS: Stars, Planets, Space operations, Cameras, Charge-coupled devices, Space telescopes, Exoplanets, Observatories, James Webb Space Telescope, Sensors
The Transiting Exoplanet Survey Satellite (TESS ) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with IC (approximately less than) 13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the star's ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.
We have studied timing properties of the Amptek Silcon Drift Detectors (SDD) using pulsed X-ray source
designed at NASA Goddard Space Flight Center. The proposed Neutron Star Interior Composition Explorer
(NICER) mission will use 56 of these detectors as X-ray sensors in an attached payload to the International
Space Station to study time variability of millisecond X-ray pulsars. Using a rastered pinhole we have measured
the delay times for single X-ray photons as a function of the impact position on the detector, as well as signal
rise time as a function of impact position. We find that the interdependence of these parameters allows us to
determine photon position on the detector by measuring the signal rise time, and, improve the accuracy of the
photon arrival time measurement.
KEYWORDS: Stars, Space operations, Charge-coupled devices, Exoplanets, Interference (communication), Sensors, Planets, Signal to noise ratio, Space telescopes, CCD image sensors
Nanosatellites, i.e. spacecraft that weigh between 1 and 10 kg, are drawing increasing interest as platforms
for conducting on-orbit science. This trend is primarily driven by the ability to piggyback nanosatellites on
the launch of large spacecraft and hence achieve orbit at greatly reduced cost. The CubeSat platform is a
standardized nanosatellite configuration, consisting of one, two, or three 10 cm x 10 cm x 10 cm units (1, 2,
or 3 "U"s) arranged in a row. We present a CubeSat-based concept for the discovery of transiting exoplanets
around the nearest and brightest Sun-like stars. The spacecraft prototype - termed ExoplanetSat - is a 3U space
telescope capable of monitoring a single target star from low Earth orbit. Given the volume limitations of
the CubeSat form factor, designing a capable spacecraft requires overcoming significant challenges. This work
presents the initial satellite configuration along with several subsystem-specific solutions to the aforementioned
constraints. An optical design based on a modified commercial off-the-shelf camera lens is given. We also
describe a novel two-stage attitude control architecture that combines 3-axis reaction wheels for coarse pointing
with a piezoelectric translation stage at the focal plane for fine pointing. Modeling and simulation results are
used to demonstrate feasibility by quantifying ExoplanetSat pointing precision, signal-to-noise ratio, guide star
magnitude, and additional design parameters which determine system performance.
We present a spectrometer design based on a novel nanofabricated blazed X-ray transmission grating which is modeled
to have superior efficiency. Here we outline a full instrument design proposed for Constellation-X which is expected to
give resolving powers ~2000 (HEW). The spectrometer advantages include lower mass budget and smaller diffractor
area, as well as order-of-magnitude more relaxed alignment tolerances for crucial degrees of freedom than reflection
grating schemes considered in the past1,2,3. The spectrometer readout is based on conventional CCD technology adapted
to operate with very high speed and low power. This instrument will enable high resolution absorption and emission line
spectroscopy in the critical band between 0.2 and 1.5 keV.
The Constellation-X Reflection Grating Spectrometer (RGS) is designed to provide high-throughput, high-resolution spectra in the long wavelength band of 6 to 50 angstrom. In the nominal design an array of reflection gratings is mounted at the exit of the Spectroscopy X-ray Telescope (SXT) mirror module. The gratings intercept and disperse light to a designated array of CCD detectors. To achieve the throughput (Aeff > 1000 cm2 below 0.6 keV) and resolution (Δλ/λ > 300 below 0.6 keV) requirements for the instrument we are investigating two possible grating designs. The first design uses in-plane gratings in a classical configuration that is very similar to the XMM-Newton RGS. The second design uses off-plane gratings in a conical configuration. The off-plane design has the advantage of providing higher reflectivity and potentially, a higher spectral resolution than the in-plane configuration. In our presentation we will describe the performance requirements and the current status of the technology development.
The Reflection Grating Spectrometer of the Constellation-X mission has
two strong candidate configurations. The first configuration, the
in-plane grating (IPG), is a set of reflection gratings similar to
those flown on XMM-Newton and has grooves perpendicular to the
direction of incident light. In the second configuration, the
off-plane grating (OPG), the grooves are closer to being parallel to
the incident light, and diffract along a cone. It has advantages of
higher packing density, and higher reflectivity. Confinement of these
gratings to sub-apertures of the optic allow high spectral
resolution. We have developed a raytrace model and analysis technique
for the off-plane grating configuration. Initial estimates indicate
that first order resolving powers in excess of 1000 (defined with
half-energy width) are achievable for sufficiently long wavelengths
(λ ≥ 12Å), provided separate accommodation is made
for gratings in the subaperture region farther from the zeroth order
location.
ECLAIRs is a microsatellite devoted to the multi-wavelength observation of the prompt emission of GRBs. For about 100 GRBs per year, ECLAIRs will provide high time resolution high sensitivity spectral coverage from a few eV up to a few hundred keV and localization to 10' in near real time. This capability is achieved by combining wide field optical and X-ray cameras sharing a common field of view with a coded-mask gamma camera providing the trigger and the localization of the bursts. ECLAIRs relies upon an international collaboration involving theoretical and hardware groups from Europe and the United States. In particular, it builds upon the extensive knowledge and expertise that is currently being gained with missions such as HETE-2 and INTEGRAL.
The ACIS instrument has been operating for three years in orbit, producing high quality scientific data on a wide variety of X-ray emitting astronomical objects. Except for a brief period at the very beginning of the mission when the CCDs were exposed to the radiation environment of the Outer van Allen Belts which resulted in substantial radiation damage to the front illuminated CCDs, the instrument has operated nearly flawlessly. The following report presents a description of the instrument, the current status of the instrument calibration and a few highlights of the scientific results obtained from the Guaranteed Observer Time.
We report on design updates for the XIS (X-ray Imaging Spectrometer)
on-board the Astro-E2 satellite. Astro-E2 is a recovery mission of Astro-E, which was lost during launch in 2000. Astro-E2 carries a total of 5 X-ray telescopes, 4 of which have XIS sensors as their focal plane detectors. Each XIS CCD camera covers a field of view of 19×19 arcmin in the energy range of 0.4-12 keV. The design of the Astro-E2 XIS is basically the same as that for Astro-E, but some improvements will be implemented. These are (1) CCD charge injection capability, (2) a revised heat-sink assembly, and (3) addition of a 55Fe radio-isotope on the door. Charge injection may be used to compensate for and calibrate radiation-induced degradation of the CCD charge transfer efficiency. This degradation is expected to become significant after a few year's operation in space. The new heat-sink assembly is expected to increase the mechanical reliability and cooling capability of the XIS sensor. The new radio-isotope on the door will provide better calibration data. We present details of these improvements and summarize the overall design of the XIS.
KEYWORDS: Charge-coupled devices, Photons, X-rays, Data modeling, Monte Carlo methods, Bismuth, Silicon, Instrument modeling, Clouds, Signal attenuation
In this work we describe a simulator developed to model the x-ray response of the backside illuminated Chandra CCDs. The model is based on the Monte Carlo technique, where individual x-ray photons are thrown into the silicon structure and the behavior of the simulated electron clouds is calculated. In order to reproduce the complex redistribution function of the device at low energies we had to introduce charge losses for the clouds formed near the illuminated surface. An important feature of this model is its simulation of Charge Transfer Inefficiency (CTI) which attempts to reproduce losses of signal charge to electron traps in the buried channel of the device during different vertical and horizontal charge transfers. The CTI model takes into account a reduction of the number of empty traps in the channel due to precursor charge packets found in the previous rows of the image section of the device. Parameters of the model were tuned to fit the large amount of experimental data acquired during the on-the-ground calibration of the Chandra devices at energies ranging from 277 eV to 9.9 keV.
We report on progress in developing low-cost methods for shaping thin-foil glass x-ray optics. Such optics might serve as substrates for reflection gratings or as foil mirrors in high-throughput missions such as Constellation-X. Novel thermal shaping to lithographically defined pin chucks leads to the desired shape with high accuracy, thereby avoiding the need for replication. To demonstrate this method we have produced 200 micron-thick glass sheets with sub-micron flatness and half power diameter below 10 arc seconds. We also present a process for depositing low-stress metallic coatings that provides high x-ray reflectivity without significant foil distortion.
KEYWORDS: Charge-coupled devices, X-rays, Signal attenuation, Image storage, Time metrology, X-ray characterization, Temperature metrology, Manganese, X-ray telescopes, Space telescopes
Front side illuminated CCDs comprising focal plane of the Chandra X-ray telescope have suffered some radiation damage in the beginning of the mission. Measurements of CTI and dark current at different temperatures led us to conclusion that the type of damage is inconsistent with the much studied type of damage created by protons with energies higher than 10 MeV. Intensive ground based investigation showed that irradiation of a CCD with low energy protons (about 100 keV) results in the device characteristics similar to the ones of the flight chips (very low dark current, the shape of the CTI temperature dependence). We were able to reliably determine that only image section of the flight chips was damaged and therefore only fast transfer from image to frame store section was affected. We have developed several techniques in order to determine the parameters of the electron traps introduced into the transfer channel of the irradiated device. One of them is based on the analysis of the amplitude of the signal in the pixels trailing the pixel that absorbed an X-ray photon of known energy. Averaging over large number of photons allowed us to get high signal/noise ratio even for pixels with extremely low signal far behind the X-ray event. Performing this analysis at different temperatures we were able to measure trap density, emission time constant, and trap cross section. Another technique is based on the analysis of the tail behind the events of very high amplitude, such as cosmic ray hits. We have developed a new scheme of clocking the device which prevents several rows of image section from being ever read out and keeps them moving back and forth. This so- called 'squeegee mode' improves CTI and can also be used to measure trap parameters, being especially effective in measuring long time constants. At least 4 different types of traps were detected, two of them with short time constant in the range from tens to a few hundred microseconds. The most damaging for the device performance are the traps with longer time constant in the millisecond range. The measurement of the trap parameters allows us to accurately model charge transfer inefficiency and helps to choose optimal operational parameters, and eventually will lead to techniques that may noticeably improve performance of a damaged CCD.
Achieving arcsecond angular resolution in a grazing-incidence foil optic X-ray telescope, such as the segmented mirror approach being considered for the Constellation-X Spectroscopy X-Ray Telescope (SXT), requires accurate placement of individual foils. We have developed a method for mounting large numbers of nested, segmented foil optics with sub- micrometer accuracy using lithographically defined and etched silicon alignment micro-structures. A system of assembly tooling, incorporating the silicon micro-structures, is used to position the foils which are then bonded to a flight structure. The advantage of this procedure is that the flight structure has relaxed tolerance requirements while the high accuracy assembly tooling can be reused. A companion paper by Bergner et al. discusses how our process could be used for the SXT. We have built an assembly truss with a simplified rectilinear geometry designed to experimentally test this alignment and mounting technique. We report results of tests with this system that demonstrate its ability to provide sub- micrometer alignment of rigid test optics.
Front side illuminated CCDs comprising focal plane of Chandra X-ray telescope have suffered some radiation damage in the beginning of the mission. Measurements of CTI and dark current at different temperatures led us to conclusion that the type of damage is inconsistent with the much studied type of damage created by protons with energies higher than 10 MeV. Intensive ground based investigation showed that irradiation of CCD with low energy protons (about 100 keV) results in the device characteristics very similar to the ones of the flight chips (very low dark current, the shape of the CTI temperature dependence). We were able to reliably determine that only image section of the flight chips was damaged and therefore only fast transfer from image to frame store section was affected. We have developed several techniques in order to determine the parameters of the electron traps introduced into the transfer channel of the irradiated device.
Soft X-ray response of X-ray Imaging Spectrometers (XIS) for the Astro-E satellite is measured with a grating spectrometer system at Osaka. First, relation between incident X-ray energy and output pulse height peak (E-PH relation) is examined with an SX grating. It is found that jump in the E-PH relation around Si-K edge is at most 2.7 eV. Second, quantum efficiency (QE) of the XIS in 0.4 - 2.2 keV range is measured relatively to the reference CCD of which absolute QE was calibrated with a gas proportional counter. The QE is fitted with a model in which CCD gate structures are considered. Systematic error on the QE results is estimated by referring an independent measurement. Third, tuning and improvement of the response function is performed. We employ six components to reproduce the response profile of the XIS. In this paper, improvement of one component which is originated in the events absorbed in the channel-stop is presented. Nevertheless, Astro-E was lost due to the launch failure. We overview the XIS project in its flight model phase, modified points of the design, problems and solutions etc., in order to be utilized in a possible recovery of the satellite.
We have performed a series of measurements with the Physikalisch-Technische Bundesanstalt beamline of the electron storage ring BESSY 1 which provide the basis for the absolute calibration of the Advanced CCD Imaging Spectrometer (ACIS). ACIS is a prime focal plane instrument aboard the recently-launched Chandra X-ray Observatory. We have achieved an absolute detection efficiency knowledge accurate to better than 5% over the 0.3 - 4 keV band. We describe our measurement and analysis techniques, including our detector response modeling and pileup corrections. We summarize a variety of external and internal consistency checks which provide the basis for our error estimates. We discuss the factors limiting the accuracy of our measurements.
The ASTRO-E X-ray Imaging Spectrometers (XISs) consists of four sets of X-ray CCD camera for the ASTRO-E mission. The XISs have been calibrated at Osaka University, Kyoto University, ISAS and MIT. The calibration experiment at Osaka focuses on the soft x-ray response of the XIS. The calibration of the XIS flight model has been performed since August 1998. We measured the signal-pulse height, the energy resolution and the quantum efficiency of the XIS as a function of energy, all of which are essential to construct the response function of the XIS. The detailed shape of the pulse-height-distribution are also investigated. We also constructed a numerical simulator of the XIS, which tracks the physical process in the CCD so as to reproduce the measured data. With a help of this simulator, we propose a model of the pulse-height-distribution of the XIS for single energy incident x-rays. The model consists of four components; two Gaussians, a constant, plus a triangle-shape component.
We have experimentally shown that heavily doped p+ layer at the silicon-silicon dioxide interface leads to charge losses in the signal electron clouds due to surface recombination and results in degraded energy resolution of the response of backside illuminated AXAF CCDs to low energy x-rays. The size of secondary ionization cloud generated by an incident x-ray photon can be much larger than that predicted from higher energy electron range-energy relations as the frontside illuminated CCD, while having high quantum efficiency at low energies. It shrinks the area of the heavily doped silicon to less than 2 percent of the pixel are, thus dramatically reducing recombination losses. If this design is combined with fully depleted silicon structures, it promises a highly efficiency x-ray sensor with a good energy resolution throughput the 0.1-15 keV band.
We report the x-ray quantum efficiency of the XIS in the soft x-ray band between 0.5 keV and 2.2 keV. We also report the x-ray and optical transmission of the OBF. We obtained the quantum efficiency of the XIS of approximately 0.25 at 0.53 keV. We also obtained the x-ray transmission of approximately 0.65 at O K(alpha) and optical transmission below 5 X 10-5 in the range 400-950 nm.
The optical chain of the spectroscopic x-ray telescopes aboard the Constellation-X spacecraft employs a reflective grating spectrometer to provide high resolution spectra for multiple spectra as a slitless spectrometer in the spectral feature rich, soft x-ray band. As a part of the spectroscopic readout array, we provide a zero-order camera that images the sky in the soft band inaccessible to the microcalorimeters. Technological enhancements required for producing the RGS instruments are described, along with prototype development progress, fabrication and testing results.
We present results from recent measurements of the channel stop structures in AXAF CCDs. We discuss refinements of a technique that uses a thin metal film with small, periodically spaced holes to restrict incident photons to well-defined regions of the pixel, providing a way to probe sub-pixel structure. By making monochromatic measurements at different energies, we can reliably determine the width and thickness of the channel stop pPLU-type silicon implant and its insulating oxide layer.
The flight AXAF CCD Imaging Spectrometer (ACIS) and the High-Resolution Mirror Assembly (HRMA) telescope were extensively calibrated at NASA MSFC's X-Ray Calibration Facility (XRCF). We present result based on HRMA/ACIS characteristics that were obtained with the following tests: (a) Point-Spread Function (PSF) tests measuring the core and wings of the PSF on-axis and at several off-axis positions, at the point of ideal focus. (b) Effective-Area (EA) test measuring the total effective collecting area over the PSF at many energies. We investigate the dependence of the effective area and energy response of ACIS/HRMA as a function of several ACIS parameters. (c) Count-rate- linearity (pile-up) test measuring the effect of increasing the mean incident rat of photons per pixel on the PSF, and EA, source centroid, and photon detection. The goal of the calibration is to provide accurate estimates of the in-orbit performance of the ACIS/HRMA instrument and to enable translation of in-orbit measurements to absolute values of the incident x-ray flux and physical models of the source emission.
Acquisition of ground calibration data from the AXAF CCD Imaging Spectrometer, one of two focal plane instruments on NASA's Advanced X-ray Astrophysics Facility, was completed in 1997. Here we summarize results of the detector level calibration effort. Our calibration program has included measurements of CCD response to undispersed synchrotron radiation, measurements of x-ray absorption fine structure, and of sub-pixel structure in the detector. Errors in the energy scale are at the level of a few tenths of one percent, and detection efficiency errors are no large than a few percent. We have also obtained new insights into the mechanisms by which the CCD gate structure and channel stops influence the CCD spectral redistribution function.
We have developed a comprehensive model of the response of a CCD to soft x-ray illumination. The model is based on the Monte Carlo technique and follows the interactions with the device material of individual photons thrown into the structure, calculating device reaction to each of them. It incorporates a very detailed description of the CCD gate structure, as well as accurately measured absorption coefficients. The fluorescent and escape peak model takes into account interactions inside the gate structure, which dramatically improves the agreement with the experimental data at energies close to the Si absorption edge. The shape of the low energy tail is simulated according to our new model of electron cloud charge splitting at the interface between Si and SiO2. An origin of the tail in the horizontally split events is explained as coming from the p+ area in the channel stop region and is modeled accordingly.
The x-ray imaging spectrometers (XIS) are x-ray CCD cameras on-board the Astro-E satellite launched in 2000. The XIS consists of 4 cameras, each of them will be installed on a focal plane of the Astro-E X-ray Telescope (XRT). The XIS not only have a higher sensitivity, which comes from a larger effective area of the XRT and thicker depletion layers of the XIS CCDs, than ASCA SIS. But also have several features that will overcome the radiation damage effects anticipated in the orbit. The calibration experiment at Osaka focuses on the soft x-ray response of the XIS. The calibration system employs a grating spectrometer which irradiates the CCD with dispersed x-rays. We have obtained preliminary results on the XIS proto model, including the energy-pulse-height relation, the energy-resolution relation, and the quantum efficiency at the energy range of 0.25-2.2 keV.
We have performed precise measurements of x-ray absorption constants for all the thin films comprising CCD gate structure, namely, phosphorous doped polysilicon, silicon dioxide, and silicon nitride. X-ray absorption of these films shows large oscillations around the corresponding absorption edges: nitrogen K, oxygen K, silicon L and K. As a result, quantum efficiency of a CCD in the soft x-ray range deviates significantly from the generally assumed simple model predictions. In order to cover the range of energies from 60 eV to 3000 eV transmission measurements were performed at several synchrotron beamlines at ALS, PTB BESSY, SRC. A model of the CCD response with near edge x-ray absorption structure taken into account predicts a very complicated shape of the energy dependence of the quantum efficiency around silicon and oxygen absorption edges. Experimental measurements of CCD quantum efficiency relative to a calibrated detector were performed at BESSY for both frontside illuminated and backside illuminated CCDs for energies around the oxygen absorption edge. Experimental results were found to be in a good agreement with our model.
We describe methods used to measure the x-ray detection efficiency of reference CCDs for the AXAF CCD imaging spectrometer in the spectral range between 2 keV and 10 keV. The reference CCDs are identical to and are used as calibration transfer standards for the actual flight CCDs. Both radioactive (55Fe) and fluorescent x-ray sources are used to illuminate the CCDs, providing a range of discrete emission lines to cover the energy band. A Si(Li) solid state detector is inserted into the beam to provide absolute calibration from 2 - 10 keV [where the Si(Li) detection efficiency can be calculated from first principles]. The response function of the Si(Li) detector is discussed, along with factors used to obtain CCD efficiency parameters from the raw data. Calibration geometry and typical data are also presented.
X-ray CCDs developed at MIT Lincoln Laboratories for the AXAF CCD Imaging Spectrometer (ACIS) have been calibrated in the 0.25 - 1.5 keV spectral range using an erect-field grating spectrometer and an electron impact x-ray source in our laboratory. A combination of the spectrometer with an x- ray CCD on high resistivity substrate allows efficient order separation, and measurement of the CCD energy scale and spectral response function as essentially continuous functions of energy over the spectrometer passband. The CCD energy scale is found to be linear to approximately 0.5% in this spectral band. Relative variations in the detection efficiency of the CCD can also be studied with this system. A near edge structure is detected in the CCD response around the oxygen K absorption edge. Comparison of these results with the data acquired with a synchrotron radiation source and monochromator confirms that these structures are features of the detector response. The edge structure allows us to measure the absolute dispersion relation of the spectrometer (and hence the zero-point of the CCD energy scale) with a precision better than 1 eV. We also detect considerable structure in characteristic nitrogen and oxygen emission lines from the electron impact source. We discuss likely mechanisms for the production of this structure.
We summarize the strategy and techniques used to calibrate x-ray CCD detectors for the AXAF CCD imaging spectrometer in the spectral range between 0.2 keV and 10 keV. The very demanding calibration requirements (energy scale knowledge error of order 0.1%; detection efficiency knowledge error of order 1%) are reviewed. The primary standards chosen for the calibration are discussed, with particular attention to the use of undispersed synchrotron radiation as a primary radiometric standard for the detection efficiency calibration. We review the basic models of the detector response which it is the objective of the calibration enterprise to constrain. The accuracy and reproducibility of the energy scale, spectral redistribution and detection efficiency calibration is discussed and illustrated with sample results from calibration of flight detectors.
The recent restructuring of the AXAF program has necessitated a review of the design of the ACIS instrument. In this paper we report on the current status of these design activities. We concentrate on changes to the baseline CCD and its impact on aspects such as the operating modes. Also we review changes to the mechanical design with respect to the passive cooling scheme facilitated by the change to a highly eccentric deep earth orbit.
A wide-field ultraviolet lens was developed under a contract from the Massachusetts Institute of Technology Center For Space Research in support of the NASA High Energy Transient Experiment (HETE). This 35 mm f/2.5 seven element lens operates over a broad portion of the near-ultraviolet spectrum and over a 52 degree field of view. Operation at cryogenic temperatures required that the lens system exhibit minimal change in focus with temperature. Aluminum was selected as the lens barrel material based on athermalization issues and the desire to minimize weight. Elastomeric bonding of elements into subcells was used for assembly along with a single adjustable airspace to compensate for tolerances.
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